This invention concerns the production of a vitreous carbon coating on a highly refractory substrate by thermally decomposing a carbon-containing organic material and depositing the carbon thus formed on the refractory substrate.
The art of coating refractory bodies, such as glass and ceramic bodies, has developed along several different lines and provides a wide diversity of properties and products. Thus, electrical resistance elements are produced on glass and ceramic cane or tubing by applying a metal, a metal oxide, or a carbon film on the surface of the body. Glass articles have also been coated with various lubricants such as fatty acids and oils. Silicones are used to render surfaces hydrophobic, and these materials, as well as varnishes and enamels, are used for electrical insulation.
One of the older coating arts is the impregnation of porous refractory bodies with carbonaceous materials such as asphalt, tar, pitch and the like, to render the body resistant to corrosive or erosive influences of a physical and/or chemical nature. The impregnated or coated body may be subsequently baked to remove a solvent and/or set the carbonaceous material. Such practices are typically illustrated in U.S. Pat. Nos. 238,806 (1881) and 1,266,335 (1918). Experience has shown that such materials are difficult to apply, and also subsequently handle, when used as coatings. Also, for many purposes, they are quite unsatisfactory in a technical sense, for example, where abrasion resistance is significant.
It is common practice to deposit a carbon coating by thermally decomposing an organic material to form carbon as a solid decomposition product. Such decomposition may be effected within the pores of a porous body after the body has been impregnated with a suitable organic material. This is a basis for solid body carbon resistors. Alternatively, the material decomposed may be a hydrocarbon gas that, in part at least, constitutes the atmosphere surrounding a body. In this case, the body serves as a substrate, and the carbon deposits within pores or on a surface depending upon the nature of the substrate body. Where a smooth surface substrate is employed, this procedure is the basis of carbon-coated resistors. As a general rule, carbon deposited after thermal decomposition is in particulate form and must be physically protected.
There is a form of carbon known as vitreous carbon. This is a hard material that is resistant to abrasion, that has an amorphous form, and that is termed "vitreous" because of its non-crystalline structure and its glass-like properties. This form of carbon is normally produced in the form of a solid body by applying a combination of high pressure and temperature to carbon in a closed chamber. However, a form of hard carbon, apparently similar in nature to vitreous carbon, has been reported in the description of an electrical resistor produced by impregnation of a porous ceramic body with carbon produced by thermal decomposition.